1. Field of the Invention
The present invention relates to a multi-passband filter. More particularly, the invention relates to a multi-passband filter comprising a dielectric member, a plurality of resonant lines provided within or on said dielectric member, and each of the resonant lines being coupled to the adjacent resonant line or lines. The multi-passband filter is for use in mobile communication apparatus.
2. Description of the Related Art
An example of a conventional antenna-duplexer unit formed by a plurality of filters in a single dielectric block is shown in FIGS. 14(A) and 14(B). FIG. 14(A) is a front view of the dielectric filters for use in the antenna-duplexer unit, and FIG. 14(B) is a longitudinal sectional view of the dielectric filters. In FIGS. 14(A) and 14(B), a dielectric block 1 has ground conductors 10 on the peripheral surfaces other than the front surface of the dielectric block 1. A plurality of resonant-line holes 31a through 31i are provided in the dielectric block 1 in which resonant lines 32a through 32i are formed, respectively. Rectangular-shaped electrodes, continuously extending from the respective resonant lines 32a through 32i, are formed on the open front surface of the dielectric block 1. Moreover, input/output-coupling electrodes 33a, 33b and 33c are inserted between the resonant-line holes 31a and 31b, between the holes 31d and 31e, and between the holes 31h and 31i, respectively, thereby capacitively coupling the adjacent rectangular electrodes. In this manner, the following types of filters are respectively formed: a band-pass filter consisting of three stages of resonators in a region indicated by F2; a band-pass filter formed of four stages of resonators in a region indicated by F3; and band-elimination filters (trap circuits), each formed of a one-stage resonator, in regions indicated by F1 and F4, respectively. Further, the input/output-coupling electrodes 33a, 33b and 33c are used as a transmitting (Tx) terminal, an antenna (ANT) terminal, and a receiving (Rx) terminal, respectively. In this manner, an antenna-duplexer unit is formed.
The above known type of antenna-duplexer unit, such as the one shown in FIGS. 14(A) and 14(B), however, presents the following problems. Either the transmitting filter or the receiving filter in this unit is adapted to reject the pass band of the other filter due to its respective band-pass filter characteristics. This requires a large number of resonator stages, which would otherwise fail to obtain a sufficient attenuation in the attenuation band, thereby inevitably enlarging the unit. One possible measure to overcome the above drawback may be to use a band-elimination filter as the transmitting filter. If, however, a multi-passband filter is formed of a single dielectric block, a transmission-line conductor is required for coupling adjacent resonators with a phase difference of .pi./2 (rad). As the transmission line, a microstripline on a dielectric should be used, and the electric length of the microstripline is accordingly longer than the length of the resonator, thereby increasing the dimensions of the space required for an array of the resonators.
Moreover, if the foregoing problem encountered by the known antenna-duplexer unit is solved simply by using a band-elimination filter as the transmitting filter, the impedance in the passband of the receiving filter, i.e., in the elimination band of the transmitting filter, as viewed from the receiving filter to the transmitting filter, becomes approximately zero. Thus, a receiving signal input from the antenna disadvantageously flows into the transmitting filter rather than the receiving filter. In order to avoid this inconvenience, a phase shifter having an electric length of .pi./2 may be provided between the transmitting filter and the antenna terminal so that the impedance viewed from the receiving filter in the stop band of the transmitting filter becomes approximately infinite. However, this requires a large number of parts, which further increases the cost.